The present invention relates to a ultra-fine inorganic fibers and a method of producing the same which are applicable to all industrial fields because they have a very large specific surface area with respect to its volume.
Inorganic particles and inorganic fibers can be utilized in every fields of industry, including glass reinforcing agents, contact lens reinforcing materials, various coating agents, part materials in the bio sensor sector, bullet-proof vests, bullet-proof helmets, part materials in the space-air sector, part materials in the electronic sector, part materials such as artificial bones, artificial vessels, etc. in the medical sector, heat resistant materials and the like.
U.S. Pat. No. 5,917,279 discloses a method of inorganic particles having a diameter of 1 to 100 nm, dispersed in a polymer binder, for the production of intermediate layers in electroluminescent arrangements. U.S. Pat. No. 6,203,768 discloses a process for producing nano-phase inorganic particles by obtaining a nano-phase metal substance embedded in a by-product phase from a mixture of a metal compound and an active material and then removing the by-product phase.
Additionally, U.S. Pat. No. 6,068,800 discloses a process for producing nano-scale particles comprising the steps of: placing a substrate on a rotatable specimen holder that is inside a reactive chamber; filling said reactive chamber with a liquid precursor solution; rotating said specimen holder; irradiating said rotating substrate and said liquid precursor solution with a laser beam; and separating said nano-scale particles from the irradiated liquid precursor solution.
The conventionally produced inorganic particles have a relatively smaller length with respect to their diameter as compared to ultra-fine inorganic fibers. That is, because they have a relatively smaller specific surface area than their volume, their effects are not good relatively when they are used as a reinforcing material.
In a paper by R. Venkatesh (Journal of the European Ceramic Society, vol. 20, 2543-2549, 2000), there is reported a method of producing alumina fibers having a diameter of more than 10 xcexcm comprising the steps of: adding SiO2 sol and polylactic acid or polyvinyl alcohol to aluminum oxychloride sol containing 30.5 weight % alumina and mixing them to thereby produce a mixture solution; spinning the mixture solution with sol to produce a gel spun fiber; and calcining the fiber at a temperature of more than 500xc2x0 C.
In this way, since only inorganic fibers having a diameter of more than 10 xcexcm have been producible by the known methods, there was a limit in increasing the specific surface area of the inorganic fibers with respect to their volume.
Accordingly, it is an object of the present invention to provide a method of producing ultra-fine inorganic fibers having a ratio of length to diameter in excess of 100 and a diameter of 10 to 1,000 nm using an electronic spinning method. In addition, it is another object of the present invention to provide ultra-fine inorganic fibers which are useful as reinforcing materials and coating materials in various fields of industry because they have a very large specific surface area with respect to their volume.
To achieve the above objects, there is provided a method of producing ultra-fine inorganic fibers according to the present invention, comprising the steps of: mixing sol or gel containing an inorganic material and thermoplastic resin solution and reacting them to produce a mixture solution thereof; electronically spinning the mixture solution under a high voltage to produce a composite fiber with the inorganic material embedded in the thermoplastic resin; and carbonating the thermoplastic resin in the composite fiber or dissolving the same in a solvent.
Additionally, the ultra-fine inorganic fibers of the present invention have a length 100 to 10,000 times larger with respect to their diameter and a diameter of 10 to 1,000 nm.
The present invention will now be described in detail with reference to the accompanying drawings.
Firstly, sol or gel containing an inorganic material is mixed and reacted with a thermoplastic resin solution to thereby produce a mixture solution thereof.
More specifically, a thermoplastic resin is dissolved in a distilled water, tetrahydrofuran, N,N-dimethylformamide or a mixed solvent thereof to thereby produce a thermoplastic resin solution. Then, the sol or gel containing the inorganic material is input and agitated to thereby produce a mixture solution thereof.
Here, the inorganic material is one of silica, ceramic, titanium, phosphor-tungsten, boron, alumina or the like. At this time, the sol or gel containing the inorganic material includes titanium isopropoxide sol or gel, aluminum alkoxyde sol or gel of the aluminum group, heteropolyacid sol or gel, silica sol or gel, ceramic sol or gel and the like.
The thermoplastic resin is one of polyvinyl acetate, polyvinyl alcohol, polylactic acid, polyamide, polyester, polypropylene and the like.
For example, in case of producing ultra-fine silica fibers, firstly, it is preferable that the molar ratio of silica gel:phosphoric acid:distilled water is adjusted to 1 to 5:0.1 to 1:10 to 80 to be agitated and a polyvinyl alcohol solution is added thereto to thereby produce a mixture solution thereof.
Next, the mixture solution is electronically spun under a high voltage to thus produce a composite fiber having an inorganic material embedded in thermoplastic resin. At this time, the electronic spinning is performed using a common electronic spinning apparatus.
Specifically, as shown in FIG. 1, the common electronic spinning apparatus comprises a main tank 1 storing a spinning dope, a metering pump 2 for constantly feeding the spinning dope, a plurality of nozzles discharging the spinning dope, a collector 4 being located at a lower end of the nozzles and collecting spun fibers, a voltage generator 6 generating a voltage and conduction apparatuses 5 transferring the generated voltage to the nozzles and the collector. The spinning dope in the main tank 1 is continuously and constantly transferred to the plurality of nozzles 3 that is given a high voltage through the metering pump 2.
Continually, the spinning dope transferred to the nozzles 3 is spun and collected on the collector 4 with a high voltage through the nozzles to thereby form a monofilament web. Preferably, the voltage required for the electronic spinning is more than 5 kV and the composite fiber having an inorganic material/thermoplastic resin produced by the electronic spinning has a diameter of less than 1,000 nm.
Next, the thermoplastic resin in the composite fiber is completely removed by carrying out a high temperature heat treatment or solvent treatment on the produced composite fiber, thereby producing an ultra-fine inorganic fiber of the present invention. The thermoplastic resin in the composite fiber is removed by being carbonated by the high temperature heat treatment or being dissolved by the solvent treatment. In this way, as the thermoplastic resin enclosing the inorganic material is completely removed, the diameter of the inorganic fiber becomes finer.
The thusly produced ultra-fine inorganic fiber of the present invention has a length 100 to 1,000 times larger than its diameter and has a diameter of 10 to 1,000 nm.
Therefore, since the ultra-fine inorganic fiber of the present invention has a very fine diameter and a very large specific surface area with respect to its volume, it is more useful for materials of various fields of industry.
It is also possible to produce inorganic particles by crushing the ultra-fine inorganic fiber of the present invention.